# Lecture 9: Joints and Articulations

## Anatomy and Physiology I

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## Learning Objectives

By the end of this lecture, students will be able to:

1. Define a joint (articulation) and classify joints structurally and functionally
2. Describe the features of fibrous joints and give examples
3. Describe the features of cartilaginous joints and give examples
4. Describe the general structure of a synovial joint
5. Classify synovial joints by shape and describe the movements each permits
6. Describe the types of movements at synovial joints
7. Describe the structure and movements of the shoulder, elbow, hip, and knee joints
8. Describe common joint injuries and disorders

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## Lecture Content

### I. Classification of Joints

#### Structural Classification (based on the material binding the bones and the presence/absence of a joint cavity)

Joints are classified structurally into three categories. **Fibrous joints** are those in which bones are joined by fibrous connective tissue, with no joint cavity present. **Cartilaginous joints** are those in which bones are joined by cartilage, also without a joint cavity. **Synovial joints** are those in which bones are separated by a fluid-filled joint cavity, making them the most movable type.

#### Functional Classification (based on the degree of movement)

Joints are also classified by how much movement they permit. **Synarthroses** are immovable joints. **Amphiarthroses** are slightly movable joints. **Diarthroses** are freely movable joints, and all synovial joints fall into this category.

### II. Fibrous Joints

**Sutures** are found only in the skull. They consist of interlocking, wavy edges of bones joined by short connective tissue fibers. Functionally, sutures are synarthroses (immovable). In old age, sutures may ossify completely, a condition called synostosis.

**Syndesmoses** are joints in which bones are connected by a ligament or interosseous membrane. Examples include the distal tibiofibular joint and the interosseous membrane between the radius and ulna. Functionally, they are amphiarthroses (slightly movable).

**Gomphoses** are peg-in-socket joints found only between the teeth and their sockets in the maxilla and mandible. They are connected by the periodontal ligament and are functionally synarthroses (essentially immovable).

### III. Cartilaginous Joints

**Synchondroses** are joints in which bones are united by hyaline cartilage. Examples include the epiphyseal plates in growing bones and the costochondral joints connecting ribs to costal cartilage. Functionally, they are synarthroses (immovable).

**Symphyses** are joints in which bones are united by fibrocartilage. Examples include the pubic symphysis and the intervertebral discs between vertebral bodies. Functionally, they are amphiarthroses (slightly movable) and are designed for strength with flexibility.

### IV. Synovial Joints — General Structure

Synovial joints are the most common and most movable type of joint in the body. All synovial joints are diarthroses (freely movable).

#### Essential Components

Every synovial joint contains several essential components. The **articular (joint) cavity** is a potential space containing synovial fluid. **Articular cartilage**, composed of hyaline cartilage, covers the ends of the articulating bones, absorbing compression and reducing friction. The **joint (articular) capsule** is a double-layered membrane enclosing the joint: the **fibrous capsule (outer layer)** is made of dense irregular connective tissue continuous with the periosteum and strengthened by ligaments, while the **synovial membrane (inner layer)** is made of loose connective tissue lining the capsule (but not covering the articular cartilage) and secreting synovial fluid. **Synovial fluid** is a viscous fluid within the joint cavity, derived from blood filtrate plus hyaluronic acid secreted by synovial membrane cells. It reduces friction as a lubricant, absorbs shock, and supplies nutrients to and removes wastes from the avascular articular cartilage. The mechanism of "weeping lubrication" involves compression of articular cartilage forcing fluid out for lubrication, with decompression reabsorbing the fluid.

#### Accessory Structures (present in some synovial joints)

Several accessory structures may be present in synovial joints. **Ligaments** are bands of dense regular connective tissue connecting bone to bone. They may be intrinsic (capsular) ligaments that are thickenings of the fibrous capsule, extracapsular ligaments outside the capsule, or intracapsular ligaments inside the capsule, such as the cruciate ligaments of the knee. **Bursae** are flattened, fluid-filled fibrous sacs lined with synovial membrane, found where ligaments, muscles, skin, tendons, or bones rub together. They reduce friction between moving structures, and their inflammation is known as **bursitis**. **Tendon sheaths** are elongated bursae wrapped around tendons subjected to friction, such as those in the wrist and fingers. **Articular discs (menisci)** are pads of fibrocartilage within certain synovial joints that improve the fit between articulating surfaces, absorb shock, and distribute weight; examples include the menisci of the knee, the disc of the TMJ, and the disc of the sternoclavicular joint. A **labrum** is a ring of fibrocartilage that deepens a socket, such as the glenoid labrum of the shoulder and the acetabular labrum of the hip. **Fat pads** cushion and protect articular surfaces, as exemplified by the infrapatellar fat pad of the knee.

<image>A cross-sectional diagram of a generalized synovial joint. Two articulating bones are shown with hyaline articular cartilage on their ends. The joint capsule surrounds the joint, with the outer fibrous capsule and inner synovial membrane clearly labeled. Synovial fluid fills the joint cavity between the cartilage surfaces. Ligaments reinforce the capsule externally. A bursa is shown between a tendon and the bone outside the capsule. An articular disc (meniscus) is shown between the bone surfaces. Labels point to each structure with brief function descriptions.</image>

### V. Types of Movements at Synovial Joints

#### Gliding

Gliding movements involve simple sliding or back-and-forth movements of flat bone surfaces, with no significant angular or rotational movement. Examples include the intercarpal joints, intertarsal joints, and joints between vertebral articular processes.

#### Angular Movements

**Flexion** decreases the angle between two bones (bending), as when bending the elbow, bending the knee, or bending forward at the hip. **Extension** increases the angle between two bones (straightening), and **hyperextension** is extension beyond the anatomical position. **Lateral flexion** is bending the trunk or head to the side. **Abduction** is movement away from the midline, and **adduction** is movement toward the midline. **Circumduction** is a cone-shaped movement combining flexion, extension, abduction, and adduction in sequence, as when "windmilling" the arm at the shoulder.

#### Rotation

Rotation is the turning of a bone around its own longitudinal axis. **Medial (internal) rotation** turns the anterior surface toward the midline, while **lateral (external) rotation** turns the anterior surface away from the midline.

#### Special Movements

Several movements are unique to particular joints. **Dorsiflexion** raises the foot toward the shin at the ankle, while **plantar flexion** points the toes downward. **Inversion** turns the sole of the foot medially (inward), and **eversion** turns it laterally (outward). **Pronation** rotates the forearm so the palm faces posteriorly, with the radius crossing over the ulna, while **supination** rotates the forearm so the palm faces anteriorly, with the radius and ulna parallel. **Protraction** moves a body part anteriorly (such as jutting the jaw forward), and **retraction** moves it posteriorly. **Elevation** lifts a body part superiorly (such as shrugging the shoulders), and **depression** lowers it. **Opposition** is the movement of touching the thumb to the tips of the other fingers on the same hand, and **reposition** returns the thumb to anatomical position from opposition.

<image>A multi-panel diagram showing types of joint movements. Panel A: Flexion and extension of the elbow — an arm bending at the elbow (flexion, decreasing angle) and straightening (extension, increasing angle). Panel B: Abduction and adduction of the arm at the shoulder — arm moving away from the body midline (abduction) and back toward it (adduction). Panel C: Rotation — a figure showing medial and lateral rotation of the humerus, with arrows indicating direction. Panel D: Special movements — four sub-panels showing dorsiflexion/plantar flexion at the ankle, pronation/supination of the forearm, inversion/eversion of the foot, and protraction/retraction of the mandible. Each movement is illustrated with arrows showing the direction of motion.</image>

### VI. Classification of Synovial Joints by Shape

**Plane (gliding) joints** have flat articular surfaces and permit short gliding movements. Examples include the intercarpal joints, the acromioclavicular joint, and vertebral facet joints.

**Hinge joints** feature a cylindrical end of one bone fitting into a trough of another, permitting flexion and extension in one plane. Examples include the elbow (humeroulnar), the knee (tibiofemoral, a modified hinge), the ankle (talocrural), and the interphalangeal joints.

**Pivot joints** have a rounded end of one bone that rotates within a ring formed by another bone and a ligament, permitting rotation. Examples include the atlantoaxial joint (C1-C2, enabling head rotation) and the proximal radioulnar joint (enabling pronation and supination).

**Condyloid (ellipsoidal) joints** feature an oval articular surface fitting into a complementary depression, permitting flexion, extension, abduction, adduction, and circumduction, but NOT rotation. Examples include the metacarpophalangeal joints (knuckles), the radiocarpal (wrist) joint, and the atlantooccipital joint.

**Saddle joints** have articular surfaces that are both concave and convex (like a saddle), permitting the same movements as condyloid joints but with greater freedom of movement. The primary example is the carpometacarpal joint of the thumb (first CMC joint), which allows opposition.

**Ball-and-socket joints** feature a spherical head of one bone fitting into a cup-shaped socket of another, permitting movement in all axes including rotation, making them the most freely movable joints. Examples include the shoulder (glenohumeral) joint and the hip (coxal) joint.

### VII. Selected Synovial Joints in Detail

#### Shoulder (Glenohumeral) Joint

The shoulder joint is a ball-and-socket joint and the most freely movable joint in the body. The head of the humerus articulates with the shallow glenoid cavity of the scapula, a design in which **stability is sacrificed for mobility**, making the shoulder the most commonly dislocated joint. Several structures contribute to its stabilization: the **glenoid labrum**, a fibrocartilage rim that deepens the glenoid cavity; the **rotator cuff muscles (SITS)** — supraspinatus, infraspinatus, teres minor, and subscapularis — whose tendons encircle and reinforce the joint capsule; the glenohumeral ligaments (which are relatively weak), the coracohumeral ligament, and the coracoacromial ligament (which forms an arch above the joint); and the long head of the biceps tendon, which passes through the joint. The shoulder joint permits flexion, extension, abduction, adduction, medial and lateral rotation, and circumduction.

#### Elbow Joint

The elbow is primarily a hinge joint but is actually a compound joint with three articulations enclosed within a single capsule. The **humeroulnar joint** is the articulation between the trochlea of the humerus and the trochlear notch of the ulna, functioning as a hinge for flexion and extension. The **humeroradial joint** involves the capitulum of the humerus and the head of the radius. The **proximal radioulnar joint** is a pivot joint in which the head of the radius rotates in the radial notch of the ulna, enabling pronation and supination. The elbow is a very stable joint, reinforced by collateral ligaments (ulnar/medial and radial/lateral) and the annular ligament, which encircles the radial head.

#### Hip (Coxal) Joint

The hip joint is a ball-and-socket joint designed for stability and weight-bearing. The head of the femur articulates with the deep acetabulum of the coxal bone. It is much more stable than the shoulder joint due to several factors: the deep socket (acetabulum) is enhanced by the **acetabular labrum**, a fibrocartilage ring; the strong joint capsule is reinforced by three thick ligaments — the iliofemoral (the strongest ligament in the body), pubofemoral, and ischiofemoral; and the **ligamentum teres (ligament of the head of the femur)** is an intracapsular ligament carrying a small artery to the femoral head. The hip joint permits flexion, extension, abduction, adduction, medial and lateral rotation, and circumduction, though with less range than the shoulder.

#### Knee (Tibiofemoral) Joint

The knee is the largest and most complex joint in the body, functioning primarily as a modified hinge joint. It includes articulations between the femoral and tibial condyles (medial and lateral compartments) and between the femur and patella (patellofemoral joint).

Important intracapsular structures include the **menisci**, C-shaped fibrocartilage pads between the femoral and tibial condyles. The medial meniscus is larger and C-shaped, attached to the tibial collateral ligament, and more commonly injured. The lateral meniscus is smaller, more circular, and less attached. Both menisci absorb shock, improve congruence, and distribute weight. The **cruciate ligaments** cross within the joint and prevent anterior-posterior displacement. The **anterior cruciate ligament (ACL)** attaches to the anterior tibia and posterior femur, preventing anterior sliding of the tibia on the femur, and is the most commonly injured knee ligament. The **posterior cruciate ligament (PCL)** attaches to the posterior tibia and anterior femur, preventing posterior sliding of the tibia.

Extracapsular ligaments include the **tibial (medial) collateral ligament (MCL)** on the medial side, which resists valgus (lateral) forces and is attached to the medial meniscus; the **fibular (lateral) collateral ligament (LCL)** on the lateral side, which resists varus (medial) forces; and the **patellar ligament**, a continuation of the quadriceps tendon extending from the patella to the tibial tuberosity. The knee primarily permits flexion and extension, with some medial and lateral rotation possible when the knee is flexed.

### VIII. Common Joint Injuries and Disorders

A **sprain** is a stretched or torn ligament, while a **strain** is a stretched or torn muscle or tendon. A **dislocation (luxation)** is the displacement of bones from their normal alignment at a joint, most common at the shoulder. A **subluxation** is a partial dislocation. **Cartilage tears**, commonly involving the menisci of the knee, may require arthroscopic surgery. An **ACL tear** is a common sports injury, typically occurring during non-contact pivoting, and often requires surgical reconstruction. **Bursitis** is inflammation of a bursa caused by repetitive motion or pressure (as in "housemaid's knee" or "student's elbow"). **Tendinitis** is inflammation of a tendon or its sheath.

**Osteoarthritis (OA)** is a degenerative joint disease and the most common form of arthritis. It involves gradual wear and tear of the articular cartilage, and bone spurs (osteophytes) may form. It primarily affects weight-bearing joints (knees, hips) and frequently used joints (fingers), with risk increasing with age, obesity, and joint injury.

**Rheumatoid arthritis (RA)** is a chronic autoimmune disease in which the immune system attacks the synovial membrane. The resulting inflammation destroys cartilage and bone, and the condition affects joints bilaterally (both hands, both knees). Pannus, the inflamed synovial tissue, erodes the articular cartilage.

**Gouty arthritis (gout)** results from the deposition of uric acid crystals in joints, classically affecting the great toe (podagra). It causes intense inflammation and pain.

<image>An anterior cross-sectional diagram of the right knee joint. The femur sits on top with its medial and lateral condyles. Between the condyles and the tibial plateau are the medial and lateral menisci (C-shaped pads). The anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) cross in the center of the joint (the ACL runs from anterior tibia to posterior femur; the PCL runs from posterior tibia to anterior femur). The tibial (medial) collateral ligament runs along the medial side and is shown attached to the medial meniscus. The fibular (lateral) collateral ligament runs along the lateral side. The patellar ligament connects the patella to the tibial tuberosity anteriorly. The joint capsule and synovial membrane line the interior. All structures are clearly labeled.</image>